docker improvement: ATUIN_DB_PASSWORD_FILE

[mortee]

It would be nice if somehow the containerized sync server could read the DB password from a file, thus enabling e.g. docker swarm's secret facility, which presents secrets to containers as files. Now I'm forced to override the entrypoint, and insert a shell command to read the pwd file and construct the DB URI, and then execute the atuin server.

I realize that this is tricky exactly because a URI needs to be constructed, but there might be some solution. Maybe ATUIN_DB_URI_FILE, and store the whole thing in the secret?